Method of solid state bonding

ABSTRACT

An article of manufacture comprised of at least two parts of electrically conductive metal joined together in overlapped relation by a solid state resistance bond at the faying interface wherein the bond area is very precisely defined by the physical shape of the attendant electrodes. The method of bonding the parts includes the utilization of a time phase application of a force pulse and an electrical energy pulse to the assembly of the part to achieve the desired solid state pulse resistance bond.

United States Patent Anderson et al.

1451 May 30, 1972 [54] METHOD OF SOLID STATE BONDING [72] Inventors:David G. Anderson; John C. Parker, both of Bloomfield Hills, Mich.

[73] Assignee: Quanta Welding Company, Troy, Mich.

[221 Filed: Jan. 8, 1971 [21] Appl. No.: 104,903

52 US. (:1 ..219/117, 219/86, 219/91, 219 119 5 1 1111. C1 ..B23k 9 00[58] FieldofSearch ....219/117,9l,78,86,119,85

[56] References Cited UNITED STATES PATENTS 1,281,454 10/1918 White..2l9/119X 2,305,042 12/1942 Thacker ..219/91 3,462,577 8/1969 Helms etal. ..2l9/86 X 3,443,055 5/1969 Gwynn et al. ..219/117 R PrimaryExaminer-J. V. Truhe Assistant Examiner-L. A. Schutzman Attorney-Wilson& Fraser [57] ABSTRACT An article of manufacture comprised of at leasttwo parts of electrically conductive metal joined together in overlappedrelation by a solid state resistance bond at the faying interfacewherein the bond area is very precisely defined by the physical shape ofthe attendant electrodes. The method of bonding the parts includes theutilization of a time phase application of a force pulse and anelectrical energy pulse to the assembly of the part to achieve thedesired solid state pulse resistance bond.

9 Claim, 7 Drawing Figures Patented May 30, 1972 2 Sheets-Sheet l 5&3 5om 2931B: women .EZD JOmPZOU N munh mwzmolmzk o m M642 6 IN mo N 292/58:gm Homo fi m .c T 47 mH D 2 WM ATTORNEYS Patented May 30, 1972 3,666,912

:2 Sheets-Sheet 2 INVENTORS DAVID G. ANDERSON JOHN C. PARKER ATTORNEYSAlthough resistance welds can be obtained in certain electricallyconductive metals without reaching the fusion point of the metal,experience to date has manifested that welds of highest strength in stopwelded structures are obtained when the metal at the interfaces hasfused. Welding conditions necessary for suitable welds in low carbonsteels, for example,

i are not extremely critical and welds of acceptable strength may beobtained over a rather wide range of current, pressure and currenttiming combinations. For equivalent heating of the metals to effect thedesired weld, the duration of the current bears an inverse relationshipto the magnitude of the current. For example, shorter current timingperiods require higher welding currents and vice versa. The tendency inresistance welding practice has been toward higher currents and shortercurrent duration periods. 7

Typically, in such welding techniques, the diameter of the electrode tiphas controlled the size of the spot weld. In practice, it has been foundthat if the diameter of the electrode, and thus the resultant weld area,is too small the resultant spot is too low in total strength and furtherproduces severe surface indentations. Also, it has been found thatslight variations in the diameter of the tip of the electrode may resultin rather substantial variations in the strength of the weld.

If the diameter of the tip of the electrode is too large, on the otherhand, abnormally high currents are required which will produce localizedheating resulting in .poor surface appearance. More importantly, from astructural standpoint, the resultant weld area may be weak because ofvoids and blowholes resulting from insufficient unit pressure. The mostimportant criterion of spot weld strength and adequate weldingconditions for a given thickness of metal is the diameter of the weldnugget. It is to be understood that the center portion of the weldnugget contributes to a minimal degree to the overall strength of theweld.

Experience has shown in the conventional resistance welding techniquesthat there is an optimum weld diameter for a given sheet thickness ifgood weld strength, maximum life, minimum sheet separation andindentation and reasonable weld strength consistency are to be obtained.Typically, weld diameters have been determined for various sheetthicknesses largely empirically. However, the relationship of welddiameter to sheet thickness (for thicknesses from 0.01 to 0.250 inches)may be expressed by the following formula:

Weld Diameter (Inches) (2.29T +0.089) where T is the thickness in inchesof one thickness (the thinner) ofthe material being welded.

In the above described techniques, the diameter of the electrode face(contacting surface) should always be somewhat larger than the expectedweld diameter to successfully contain the molten weld metal and theminimize expulsion and indentation and, also, to minimize electrodedeformation.

It will be manifest that, in view of the above comments, there is adefinite limitation to the size of a weld which can be achieved with theknown welding techniques.

SUMMARY The present invention is concerned with the production of anarticle of manufacture consisting of at least two juxtaposedelectrically conductive parts having a definitively shaped solid statepulse resistance bond interconnecting the parts which wherein the zoneof bonding includes a thin layer of thermally modified material.

The objectives of theinvention are typically achieved by utilizing apair of electrodes provided with raised regions on the workingcontacting faces thereof which are maintained in registry on theopposite sides of the juxtaposed parts, and then subjected to atime-phased application of pressure and high current density pulses toproduce a weld interconnecting the parts having negligible indentationon the exposed surface of the parts and having a shaped weld zone ofminimal depth.

The shaped weld is determined by the shape of the raised portions of thecontacting faces of the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects of theinvention will become manifest to those skilled in the an from readingthe following detailed description of an embodiment of the inventionwhen considered in the light of the accompanying drawings in which:

FIG. 1 is a flow diagram of the method of the invention;

FIG. 2 is a schematic diagram of 'one form of the apparatus andassociated system for accomplishing the method illustrated in FIG. 1; I

FIG. 3 is an enlarged fragmentary view of the electrodes and work placesbeing bonded together as illustrated in FIG. 3;

FIG. 4 is an exploded view of the electrodes illustrated in FIG. 3; and

FIG. 5, 6 and 7 illustrate typical weld shapes which can be effectivelyachieved by the method of the invention to produce new and usefularticles of manufacture.

DESCRIPTION OF PREFERRED EMBODIMENT As diagrammatically illustrated inFIG. 1, there is shown in flow diagram form the process of the inventionas applied to the production of an article of manufacture typicallycomprised of two sheets of electrically conductive material bondedtogether by a shaped weld. The process basically involves thetime-phased application of a force pulse and a pulse of electricalenergy to the parts being bonded to effect a current density at thefaying interface within the range of from 500,000 to 3,000,000 amperesper square inch at the faying interface of parts being bonded.

To completely understand the process of the invention, as illustrated inFIG. 1, reference will be made to the bonding of two sheets ofelectrically conductive materials as specifically illustrated in FIGS.2,3 and 4. At the outset or commencement of the procedure, two parts 10and 12 of sheet material such as, for example, 1010 mild steel ofathickness of 0.032 inch are disposed in overlapping superposed relationbetween a pair of cooperating relatively movable electrodes 14 and 16.The uppermost electrode 14 is mounted on an upper platen or fixedsupport 18 and is electrically insulated therefrom by a layer ofinsulating material 20. The lowermost electrode 16 is mounted forunitary movement on a lower platen 22 and is electrically insulatedtherefrom by a layer of insulating material 24. The lower platen 22 iscoupled to a force mechanism 30 which may provide for selectivereciprocating movement of the electrode 16 relative to the electrode 14to initially enable the disposition of the parts 10 and I2 therebetween.At such stage in the operation, the electrodes 14 and 16 are movedtogether until the assemblage of the parts 10 and 12 are in relativelyfixed position. The electrodes 14 and 16 are provided with outwardlyextending projections I4 and 16', respectively, which are positioned inregistry such that they are in direct alignment when the associatedelectrodes are in work contacting relation. For use on titanium alloys,for example, the extent of the projections 14' and 16' may be of theorder of 0.001 to 0.002 inches which effectively minimizes expulsion andenhances appearance of the finished weld.

The force mechanism 30, which typically includes a pressure transducer,is effective to apply a force pulse on the lower platen 22, which issuperimposed on the initial forces applied by the closing of theelectrodes 14 and 16 by the relative closing movement of the associatedplatens l8 and 22. The specific mechanism employed for developing theforce by the force mechanism 30 may be the type illustrated anddescribed in the U.S. Patent to A.G. Vang U.S. Pat. No. 3,059,094 issuedOct. 16, 1962. It will be understood that in time phased relation withthe application of a force pulse on the sheets 10 and 12 by the forcemechanism 30, an electrical energy pulse is applied to the electrodes I4and 16, as will be explained in greater detail hereinafter.

1 It has been found that in practice, pressures developed of the orderof from 1,000 to 10,000 pounds have been employed to producesatisfactory solid state welds with the described process. Thesepressures are not considered to be critical and may be varied over arather wide range. The pressures imposed on the system can be imposed insinusoidal wave form, and, typically, the force pulse is initiated firstand before the force pulse reaches the maximum amplitude, the electricalenergy pulse is commenced. Typically, the electrical energy pulse isthen allowed to fully decay before the full decay of the force pulse.The electrical energy pulse is developed in the system illustrated inFIG. 2 in the secondary winding of a pulse weld transformer 32 which hasits power switch 36. The weld power supply 34 typically includes a bankof capacitors and a charging circuit which are effective to produce aninstantaneous source of electrical energy to the pulse weld transformer32 as will be explained in greater detail hereinafter.

The force mechanism 30 is coupled to a force mechanism power supply 38through a force power switch 40.

The weld power switch 36 and the force power switch 40 are controlled intimed relation to one another by a process control unit 42. The controlunit 42 is effective to energized the respective power switches 36 and40 in such a fashion that, typically, the force power switch 40 isenergized to commence the application of force by the lower platen 22 toapply a force pulse at the faying interface of the sheets 10 and 12.Then, the control unit 42 is effective to energize the weld power switch36 to allow the capacitors of the weld power supply 34 to discharge andproduce an electrical energy pulse in the primary winding of the pulseweld transformer 32. The secondary winding of the pulse weld transformer32 causes a high electrical energy pulse between the electrodes 14 and16 and the sheets 10 and 12. An electrical energy pulse having a currentdensity of the order of from 500,000 to 3,000,000 amperes per squareinch of weld interface has been satisfactory for achieving the desiredresults of the invention of obtaining a solid state bond. In operationof the illustrated embodiment, the force pulse peaks in the order offrom 0.5 to 2.0 milliseconds before electrical energy peaks.

It has been theorized that the phenomenon involved in the weldingprocess of the invention involves electrical energy at the interface ofthe sheets 10 and 12 in magnitude sufficient to establish atomic bondsacross the interface, resulting in a solid state pulse resistance weld.The electrical energy pulse applied by the pulse transformer 32 followsan electrical path through the electrode 14, the sheets 10 and 12 andthe electrode 16.

The control unit 42 is typically energized after the lower platen 22 andthe upper platen 18 are initially closed to a point where the sheets 10and 12 are firmly held between terminal ends of the electrodes 14 and16. The energized control unit 42 initially signals the force switch 40'to couple the force mechanism 30 to its power supply 38 to effectivelyimpart a force pulse of 8.0 millisecond duration, for example, with apeak force of approximately 5,000 pounds to the platen 22, causing thesheets 10 and 12 to be bonded to be forced tightly against the adjacentoverlapped surfaces.

Typically, the control unit 42 is programmed to energize the weld powerswitch 36 coupling the pulse transformer 32 to the welding power supply38 to thereby apply an electrical energy pulse to the electrode elements14 and 16. The energization thereof commences, in the describedembodiment, prior to the instant the force pulse reaches its peakamplitude.

The temperature of the overall mass of the bonded members is raised onlyslightly during the bonding process, for example, to F., while at theinterface where high electrical resistance exists, there is a smalllocalized area of substantial thinness or surface skin which mayapproach melting or forging temperatures. The energy requirements aretypically only 10 to 20 per cent of the magnitude of the energyrequirements of conventional welding processes.

Since the solid state pulse resistance bonding procedure described aboveis capable of joining metal parts without significant melting andresultant change in the crystalline structure of the metal, and capableof more exactly controlling and determining the electrical current path,very precisely shaped weld configurations can be achieved. FIG. 4, forexample, shows an annularly shaped weld. Such a weld consumes lessenergy to effect than a spot weld, for example, wherein the en tirecircular area is welded. A weld configuration such as illustrated inFIG. 4 exactly conforms to the configuration of the cooperatingelectrode projections 14' and 16' and is equally as strong as a weldwhere the entire curcular area is fused and welded. It has been foundempirically that the center portion of a complete circular area weld isof little consequence when considering the overall weld strength andcan, in many instances, be completely removed by drilling for exampleand not change the overall weld strength characteristics.

Having the capability of controlling the current path between theprojections 14' and 16' of the electrodes 14 and 16, respectively, manydifferently shaped projections can be formed to provide for variouscorresponding shaped welds. Typical shapes of the electrode projectionscan achieve weld shapes of the types illustrated in FIGS. 5, 6 and 7.

In one typical embodiment, it has been found in welding two sheets of atitanium alloy together wherein the alloy contained of the order of 6percent aluminum and 4 percent vanadium of a thickness of 0.040 inch, aweld having an CD. of 0.500 inch and an ID. of 0.400 inch was formedhaving a tensile shear strength of more than 5,000 pounds. Aconventional resistance or spot weld of maximum size for the samematerial would exhibit a tensile shear strength of approximately 2,500pounds. Accordingly, it is readily apparent that the present inventioncan produce a weld of larger diameter. than conventionally achievedwelds which inherently results in greatly enhanced mechanical propertiessuch as tensile strength and fatigue strength, for example.

It has been found that the weld shapes produced by the shaped electrodesprojections conform rather exactly to the shape of the cooperatingelectrode projections. As pointed out herein above, in the conventionalresistance welding techniques the diameter or shape of the electrode isempirically determined and is usually larger in diameter than thediameter of the resultant weld.

7 While mention has specifically been made to the effect that theelectrode elements employed for carrying out the method of the inventionin producing the desired composite have been found to have projectionsor extensions therefrom of the desired shape, satisfactory results canbe achieved by shaping the entire electrode with the cross-sectionalconfiguration of the desired bond to be formed. v

In accordance with the provisions of the patent statutes, we haveexplained the principle and mode of operation of our invention and haveillustrated and described what we now consider to represent its bestembodiment. However, we desire to have it understood that the inventionmay be practiced otherwise than as specifically illustrated anddescribed without departing from its spirit or scope.

What we claim is:

1. A method of producing a sharply defined bond between at least twoparts of electrically conductive material comprismg:

providing a first surface of one of said parts;

providing a second surface of the other of said parts;

engaging said first and second surfaces;

providing a pair of opposing registering shaped electrodes on oppositesides of said first and second surfaces;

imposing a force pulse on the faying interface between said first andsecond surfaces to urge said surfaces into intimate contact; and

applying an electrical energy pulse between said electrodes to produce acurrent density within the range of from 500,000 to 3,000,000 amperesper square inch of faying interface between said first and secondsurfaces desired to be welded in time-phased relation with said forcepulse to produce a sharply disposed solid state resistance bond form anintegral bonded energy pulse is of a duration of from 0.5 to 5.5milliseconds.

6. The method defined in claim 1 wherein the force pulse applied to saidparts arrives at its peak amplitude before the electrical energy pulsearrives at its peak magnitude.

7. The method as defined in claim 1 wherein at least one of said partsis a metal sheet.

8. The method as defined in claim 3 wherein said shaped projections onsaid cooperating electrodes are annular.

9. The method as defined in claim 1 wherein said force pulse peaks aftersaid electrical energy pulse peaks.

1. A method of producing a sharply defined bond between at least twoparts of electrically conductive material comprising: providing a firstsurface of one of said parts; providing a second surface of the other ofsaid parts; engaging said first and second surfaces; providing a pair ofopposing registering shaped electrodes on opposite sides of said firstand second surfaces; imposing a force pulse on the faying interfacebetween said first and second surfaces to urge said surfaces intointimate contact; and applying an electrical energy pulse between saidelectrodes to produce a current density within the range of from 500,000to 3,000,000 amperes per square inch of faying interface between saidfirst and second surfaces desired to be welded in timephased relationwith said force pulse to produce a sharply disposed solid stateresistance bond to weld said surfaces together to form an integralbonded article.
 2. The method defined in claim 1 wherein at least one ofsaid shaped electrodes is provided with a shaped projection extendingfrom the distal portion thereof.
 3. The method defined in claim 1wherein said shaped electrodes are provided with shaped extensionsextending from the distal portions thereof in registry with one another.4. The method as defined in claim 1 wherein said electrical energy isapplied as a single pulse.
 5. The method as defined in claim 4 whereinsaid electrical energy pulse is of a duration of from 0.5 to 5.5milliseconds.
 6. The method defined in claim 1 wherein the force pulseapplied to said parts arrives at its peak amplitude before theelectrical energy pulse arrives at its peak magnitude.
 7. The method asdefined in claim 1 wherein at least one of said parts is a metal sheet.8. The method as defined in claim 3 wherein said shaped projections onsaid cooperating electrodes are annular.
 9. The method as defined inclaim 1 wherein said force pulse peaks after said electrical energypulse peaks.